The present invention relates generally to an optical apparatus with a mirror and a mirror holder, and more particularly to an exposure apparatus as an optical apparatus used for fabricating semiconductor and the like.
A semiconductor exposure apparatus transfers circuit patterns from an original form (or reticle) onto a substrate (or a silicon wafer). A projecting optical system uses a light source of ultra-high pressure mercury lamp (i-lay), KrF excimer laser, or ArF excimer laser, and a lens for imaging a reticle's pattern onto a wafer in these light's wavelength.
To fabricate high integrated circuits, a projecting lens requires high resolution. Thus, a lens for a semiconductor exposure apparatus is fabricated with lower aberration.
However, in recent teachings of the fine development of a semiconductor circuit, the exposure light's wavelength, in an exposure apparatus, has been shortened. Hence, Extreme Ultraviolet Radiation (hereinafter EUV light) will be used as exposure light (Japanese Patent Application No. 2003-172857).
For EUV light, a mirror as an optical system is used because a lens cannot be used as an optical system since it does not transmit EUV light. As a result, a developing mirror in EUV light does not have reflection rates greater than 70%. Thus, the mirror's temperature increases because it absorbs about 30% of the EUV light and changes it into heat. The increasing temperature causes bad aberration due to thermal expansion and extreme deformation of the mirror. Therefore, a mirror cooling apparatus is arranged to inhibit thermal distortion and the bad aberration. However, although it is able to inhibit rising temperature, it is not able to absolutely prohibit the rising temperature in the whole mirror from deforming the mirror. When deformation of the mirror results from simple expansion or contraction, correction such as to a mirror's position can decrease the bad aberration because the main change is only to the curvature. However, the mirror cannot have simple expansion or contraction because the symmetry axis of an illuminating area on the mirror, in this kind of conventional mirror holder, differs extremely from that of the mirror holder.
Accordingly, it is an exemplary object of the present invention to provide a mirror holding method for inhibiting asymmetry surface deformation which extremely degrades optical performance, illumination point transference, and defocus value; and for changing deformation value of the mirror into simple deformation, by eliminating thermal stress, to inhibit the above bad aberration.